Reiner's assessment: these things have no tactics at all. Depending on how fast they climb, he'll just … scoop Eren up and go to another tree. But in the meantime:

Eren will continue burning! He'll try to direct the fire from the weapon to the slime itself. Does the slime appear hurt by the fire, or just confused and flailing?

Reiner will attempt to bury Slime AE in its newly formed pit. Will it hold? Who knows! But hopefully it'll get the slime out of the way for a while.